I. Field of the Invention
The present invention relates generally to object position determination using satellites. More specifically, the present invention relates to a method for resolving ambiguity in an ambiguous position solution for a user terminal in a satellite communications system.
II. Related Art
A typical satellite-based communications system comprises at least one terrestrial base station (hereinafter referred to as a gateway), at least one user terminal (for example, a mobile telephone), and at least one satellite for relaying communications signals between the gateway and the user terminal. The gateway provides links from a user terminal to other user terminals or communications systems, such as a terrestrial telephone system.
A variety of multiple access communications systems and techniques have been developed using time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA), the basics of which are well known in the art. The use of CDMA techniques in multiple access communications systems is disclosed in U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990, entitled "Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters", and U.S. patent application Ser. No. 08/368,570, filed Jan. 4, 1995, entitled "Method and Apparatus for Using Full Spectrum Transmitted Power in a Spread Spectrum Communication System for Tracking Individual Recipient Phase Time and Energy", which are both assigned to the assignee of the present invention, and are incorporated herein by reference.
The above-mentioned patent documents disclose multiple access communications systems in which a large number of generally mobile or remote system users each employ at least one user terminal to communicate with other system users or users of other connected systems, such as a public telephone switching network. The user terminals communicate through gateways and satellites using CDMA spread-spectrum type communications signals.
Communications satellites form beams which illuminate a "spot" or area produced by projecting satellite communications signals onto the Earth's surface. A typical satellite beam pattern for a spot comprises a number of beams arranged in a predetermined coverage pattern. Typically, each beam comprises a number of so-called sub-beams (also referred to as CDMA channels) covering a common geographic area, each occupying a different frequency band.
In a typical spread-spectrum communications system, a set of preselected pseudorandom noise (PN) code sequences is used to modulate (i.e., "spread") information signals over a predetermined spectral band prior to modulation onto a carrier signal for transmission as communications signals. PN code spreading is a method of spread-spectrum transmission that is well known in the art, produces a signal for transmission that has a bandwidth much greater than that of the data signal. In a forward communications link (that is, in a communications link originating at a gateway and terminating at a user terminal), PN spreading codes or binary sequences may be used to discriminate between signals transmitted by different satellites or gateways or over different beams, and to resolve multipath signals. PN spreading codes are typically shared by communications signals within a given cell or sub-beam.
In a typical CDMA spread-spectrum system, channelizing codes are used to differentiate signals intended for various user terminals that are transmitted within a satellite beam on the forward link. That is, a series of unique orthogonal channels, including pilot and paging signal channels, are provided for transmission of information to user terminals on the forward link by using unique "channelizing" orthogonal codes. Walsh functions are generally used to implement the channelizing codes.
Typical CDMA spread-spectrum communications systems, such as disclosed in U.S. Pat. No. 4,901,307, contemplate the use of coherent modulation and demodulation for forward link user terminal communications. In communications systems using this approach, a "pilot" carrier signal (hereinafter referred to as a "pilot signal") is used as a coherent phase reference for forward links. That is, a pilot signal, which contains no data modulation, is transmitted by a gateway throughout a region of coverage. A single pilot signal is typically transmitted by each gateway for each beam used for each frequency used, that is, each sub-beam or CDMA channel. These pilot signals are shared by user terminals receiving signals from the gateway.
While each beam, or sub-beam, can be said to have a unique pilot signal (subject to system wide re-use), they may not be generated using different PN code polynomials, but can use the same spreading code with different code phase offsets. This allows PN codes that can be readily distinguished from each other, in turn distinguishing originating gateways or satellites, and beams or sub-beams. In the alternative, a series of PN spreading codes are used within the communication system with different PN codes being used for each gateway or satellite plane through which gateways communicate, and timing offsets used for each beam or sub-beam. It will be readily apparent to those skilled in the art that as many or as few PN codes as desired can be assigned to identify specific signal sources in the communication system, subject to complexity, availability, and system capacity limitations.
Pilot signals are used by user terminals to obtain initial system synchronization and time, frequency, and phase tracking of other signals transmitted by the gateway. Phase information obtained from tracking a pilot signal carrier is used as a carrier phase reference for coherent demodulation of other system signals or traffic signals. This technique allows many traffic signals to share a common pilot signal as a phase reference, providing for a less costly and more efficient tracking mechanism.
When a user terminal is not involved in a communications session (that is, the user terminal is not receiving or transmitting traffic signals), the gateway can convey information to that particular user terminal using a signal known as a paging signal. For example, when a call has been placed to a particular mobile phone, the gateway alerts the mobile phone by means of a paging signal. Paging signals are also used to distribute system overhead information.
A user terminal can respond to a paging signal by sending an access signal or access probe over the reverse link (that is, the communications link originating at the user terminal and terminating at the gateway). The access signal is also used when a user terminal originates a call. Access probes or signals may use their own sets of PN code sequences for spreading on the reverse link, providing a form of signal identification which can restrict the specific satellites or gateways that can receive and process such signals.
When communications are required with a user terminal, the communications system may need to determine the position of the user terminal. The need for user terminal position information stems from several considerations. One consideration is that the system should select an appropriate gateway for providing the communications link. For example, gateways in communication with satellites that are well elevated above a user terminal horizon may provide higher quality communication links. It is desirable to use a gateway that is in communication with such satellites. Therefore, when communications are required with a particular user terminal, the communications system needs to know the position of the user terminal, relative to various satellites, in order to select the appropriate gateway.
Another consideration is allocation of a communications link to the proper service provider (for example, a telephone company). A service provider is typically assigned a particular geographic territory, and handles all calls with users in that territory. When communications are required with a particular user terminal, the communications system can allocate the call to a service provider based on the territory within which the user terminal is located. In order to determine the appropriate territory, the communications system requires the position of the user terminal. A similar consideration arises when calls must be allocated to service providers based on political boundaries.
An important requirement in position determination for a satellite-based communications system is speed. When communications are required with a particular user terminal, the gateway that will serve the user terminal should be selected rapidly. For example, a mobile telephone user is not likely to tolerate a delay of more than a few seconds when placing a call. The desire for positioning accuracy is less important than the desire for speed; an error of less than 10 kilometers (km) is considered adequate, in order to achieve a short delay. In contrast, most conventional approaches to satellite-based position determination emphasize accuracy over speed.
Further, many conventional approaches result in ambiguous position solutions. That is, a determined position solution includes more than one possible position for a user terminal. What is needed, therefore, is a system and method for resolving the ambiguity in an ambiguous position solution.